This invention relates to seals for sealing a circumferential gap between two machine components that are relatively rotatable with respect to each other, and, more particularly, to a hybrid brush seal having a plurality of bristles that are mounted in a ring shape on a first machine component and that have bristle ends directed at sealing surface of the second, rotating machine component, whereby the bristle ends are titled at an angle in the circumferential direction in the annular plane of the bristles, primarily in the direction of shaft rotation. The bristle ends are kept from direct contact with the rotating machine component via one or more shoes which are designed such that as the shaft rotates a hydrodynamic film will separate the shoe from the shaft. The shoe(s) is attached to the bristle ends at discreet locations.
Turbomachinery, such as gas turbine engines employed in aircraft, currently is dependent on either labyrinth (see FIGS. 1A-1E), brush (see FIGS. 2A and 2B) or carbon seals for critical applications. Labyrinth seals provide adequate sealing, however, they are extremely dependent on maintaining radial tolerances at all points of engine operation. The radial clearance must take into account factors such as thermal expansion, shaft motion, tolerance stack-ups, rub tolerance, etc. Minimization of seal clearance is necessary to achieve maximum labyrinth seal effectiveness. In addition to increased leakage if clearances are not maintained, such as during a high-G maneuver, there is the potential for increases in engine vibration. Straight-thru labyrinth seals (FIG. 1A) are the most sensitive to clearance changes, with large clearances resulting in a carryover effect. Stepped labyrinth seals (FIGS. 1B and 1C) are very dependent on axial clearances, as well as radial clearances, which limits the number of teeth possible on each land. Pregrooved labyrinth seals (FIG. 1D) are dependent on both axial and radial clearances and must have an axial clearance less than twice the radial clearance to provide better leakage performance than stepped seals.
Other problems associated with labyrinth seals arise from heat generation due to knife edge to seal land rub, debris from hardcoated knife edges or seal lands beings carried through engine passages, and excessive engine vibration. When seal teeth rub against seal lands, it is possible to generate large amounts of heat. This heat may result in reduced material strength and may even cause destruction of the seal if heat conducted to the rotor causes further interference. It is possible to reduce heat generation using abradable seal lands, however, they must not be used in situations where rub debris will be carried by leakage air directly into critical areas such as bearing compartments or carbon seal rubbing contacts. This also holds true for hardcoats applied to knife edges to increase rub capability. Other difficulties with hardcoated knife edges include low cycle fatigue life debits, rub induced tooth-edge cracking, and the possibility of handling damage. Engine vibration is another factor to be considered when implementing labyrinth seals. As mentioned previously, this vibration can be caused by improper maintenance of radial clearances. However, it can also be affected by the spacing of labyrinth seal teeth, which can produce harmonics and result in high vibratory stresses.
In comparison to labyrinth seals, brush seals can offer very low leakage rates. For example, flow past a single stage brush seal is approximately equal to a four knife edge labyrinth seal at the same clearance. Brush seals are also not as dependent on radial clearances as labyrinth seals. Leakage equivalent to approximately a 2 to 3 mil gap is relatively constant over a large range of wire-rotor interferences. However, with current technology, all brush seals will eventually wear to line on line contact at the point of greatest initial interference. Great care must be taken to insure that the brush seal backing plate does not contact the rotor under any circumstances. It is possible for severing of the rotor to occur from this type of contact. In addition, undue wire wear may result in flow increases up to 800% and factors such as changes in extreme interference, temperature and pressure loads, and rubbing speeds must be taken into account when determining seal life.
The design for common brush seals, as seen in FIGS. 2A and 2B, is usually an assembly of densely packed flexible wires sandwiched between two plates. The free ends of the wires protrude beyond the plates and contact a land or runner, with a small radial interference to form the seal. The wires are angled so that the free ends point in the same direction as the movement of the runner. Brush seals are sized to maintain a tight diametral fit throughout their useful life and to accommodate the greatest combination of axial movement of the brush relative to the rotor.
Brush seals may be used in a wide variety of applications. Although brush seal leakage generally decreases with exposure to repeated pressure loading, incorporating brush seals where extreme pressure loading occurs may cause a xe2x80x9cblow overxe2x80x9d condition resulting in permanent deformation of the seal wires. Brush seals have been used in sealing bearing compartments, however coke on the wires may result in accelerated wear and their leakage rate is higher than that of carbon seals.
One additional limitation of brush seals is that they are essentially unidirectional in operation, i.e., due to the angulation of the individual wires, such seals must be oriented in the direction of rotation of the moving element as depicted by the arrow in FIG. 2A. Rotation of the moving element or rotor in the opposite direction, against the angulation of the wires, can result in permanent damage and/or failure of the seal. In the particular application of the seals required in the engine of a V-22 Osprey aircraft, for example, it is noted that during the blade fold wing stow operation, the engine rotates in reverse at very low rpm""s. This is required to align rotor blades when stowing wings. This procedure is performed for creating a smaller aircraft footprint onboard an aircraft carrier. Reverse rotation of the engine would damage or create failure of brush seals such as those depicted in FIGS. 2A and 2B.
One attempt to limit wear of brush seals is disclosed in U.S. Pat. No. 5,026,252 to Hoffelner in which a sliding ring is interposed between the bristle pack of the seal and the moving element or rotor to avoid direct contact therebetween. The bristle ends are received within a circumferential groove in the sliding ring and are allowed to freely float or move within such groove. Although bristle wear may be reduced in this design, it is believed that the seal created at the interface of the sliding ring and rotor is unsatisfactory.
Carbon seals are generally used to provide sealing of oil compartments and to protect oil systems from hot air and contamination. Their low leakage rates in comparison to labyrinth or brush seals are well-suited to this application, however they are very sensitive to pressure balances and tolerance stack-ups. Pressure gradients at all operating conditions and especially at low power and idle conditions must be taken into account when considering the use of carbon seals. Seals must be designed to have a sufficiently thick seal plate and the axial stack load path must pass through the plate as straight as possible to prevent coning of the seal. Another consideration with carbon seals is the potential for seepage, weepage or trapped oil. Provisions must be made to eliminate these conditions which may result in oil fire, rotor vibration, and severe corrosion
According to the Advanced Subsonic Technology Initiative as presented at the NASA Lewis Research Center Seals Workshop, development of advanced sealing techniques to replace the current seal technologies described above will provide high returns on technology investments. These returns include reducing direct operating costs by up to 5%, reducing engine fuel burn up to 10%, reducing engine oxides of emission by over 50%, and reducing noise by 7 dB. For example, spending only a fraction of the costs needed to redesign and re-qualify complete compressor or turbine components on advanced seal development can achieve comparable performance improvements. In fact, engine studies have shown that by applying advanced seals techniques to just a few locations can result in reduction of 2.5% in SFC.
It is therefore among the objectives of this invention to provide a hybrid brush seal for sealing a circumferential gap between two relatively rotating machine components such as a rotor and a stator which exhibits low wear rates, which provides low leakage, which has low dependence on clearances between the rotor and stator to retain acceptable sealing characteristics and which has a low contamination potential.
These objectives are accomplished in a hybrid brush seal according to this invention which comprises a number of individual seal bristles each affixed at an outer end to the stationary machine component or stator in an annular shape corresponding to the circumferential gap between the stator and the rotating component or rotor. The opposite, inner end of the seal bristles engage one or more shoes located with respect to the rotor to create a seal therebetween while avoiding contact of the seal bristles with the rotor.
This invention is predicated upon the concept of employing some of the advantages of conventional brush seals in an improved, hybrid brush seal which overcomes typical brush seal limitations of wear and unidirectional operation. In the presently preferred embodiment, each of the shoes is connected at discrete points to the abutting seal bristles such that the leading edge of the shoe is oriented to have less contact with the rotor than the trailing edge of the shoe. In one embodiment, each shoe is connected at two spaced locations to the abutting seal bristles by electron beam welding or similar mounting techniques, thus creating two hinge points for the shoe to translate about.
In operation, the shoe functions very similarly to that of a tilting pad bearing shoe. Prior to rotation of the rotor, the shoe is in contact with the rotor surface. Because the leading edge of the shoe has less contact with the rotor than its trailing edge, when the rotor begins to rotate a hydrodynamic wedge is created that lifts the shoe slightly off of the surface of the rotor. Consequently, the shoe xe2x80x9cfloatsxe2x80x9d over the rotor at a design gap, such as 0.0005 to 0.0010 inches.
The advantages of this hybrid brush seal are many. It will have the same sealing characteristics of existing brush seals, but will never change in performance due to bristle wear. The brush seal backing plate can be moved further outboard of the I.D. because the shoe will prevent the bristles from bending over in high pressure applications. Each shoe will be designed to have a certain amount of interference with the shaft prior to rotation. Thus, the seal can be slightly off center during assembly but once rotation begins, each pad will lift-off. Hence, tight tolerances can be relaxed.
The hybrid seal can be utilized in all seal applications, labyrinth, brush and carbon. The robust design will eliminate the careful handling now required of carbon seals utilized in lube system compartments. This seal may allow the engine designer to utilize less parts in the assembly as this seal will permit xe2x80x9cblindxe2x80x9d assemblies to occur.
The following table provides a comparison of the seal of the subject invention with currently available technology.